SUMMARY There is a fundamental gap in the knowledge base about how chronic manganese exposures impacts develop- ment of Alzheimer?s disease. The neurotoxic effects of manganese poisoning are known, as well as the motor impairments that are its behavioral sequelae. However, chronic lower-level exposures have not been studied. The neuropathology of Alzheimer?s disease develops over decades prior to onset of severe cognitive and be- havioral change (dementia) and thus its development is particularly susceptible to influence from environmental factors. Manganese represents an environmental toxin with high likelihood of importance since exposure occurs through multiple sources (contaminated water, food, inhalation from pollution and industrial complexes). Further, exposure directly targets many of the primary mechanisms involved in Alzheimer?s disease pathology: ?-amyloid accumulation, oxidative stress and glial changes relating to neuroinflammation. Our central hypothesis is that Chronic elevated manganese (Mn) exposure drives cognitive decline through impaired glutamate homeostasis. Our long-term objectives are to isolate the direct link(s) between Mn and cognitive decline by demonstrating how chronic Mn exposure affects altered glutamate clearance and other pathologies to a greater extent in mouse and human stem cell models of AD than in controls. We will do this by: (1) Demonstrating the extent to which chronic Mn exposure accelerates AD neuropathology. Following 3 months treatment with Mn to significantly elevate brain Mn we will assess multiple markers of AD-related neuropathology, oxidative stress and neuroin- flammation at the gene, protein and cellular level incorporating direct hypothesis testing and hypothesis gener- ating approaches. Changes will be assessed prior to- and after onset of significant ?-amyloid accumulation (6- and 12 months of age), and in ?-amyloid positive (APP/PSEN1, familial AD model) and negative mice (APOE4/TREM2, sporadic AD model; and wild-type mice). (2) Demonstrating the extent to which chronic Mn exposure impacts cognitive decline. We will assess learning and memory at the two age points using a com- prehensive battery of behavioral tests for cognitive and motor changes. We will directly assess the potential for Mn to impact the molecular basis of memory, synaptic strengthening through long term potentiation. Human stem cell models will be utilized to validate these findings. (3) Establishing the role of brain Mn levels in synaptic glutamate homeostasis. We will address the hypothesis that Mn directly impacts synaptic glutamate homeostasis through primary cell culture and stem cell models and assess glutamate uptake and release. We will functionally test the glutamatergic system by electrophysiological recordings. Finally we will utilize GLT-1 knockout mice to further probe the role of GLT-1 in particular in this relationship. Together these data will confirm the role of chronic Mn exposure in AD neuropathology and cognitive decline, and specifically address its impact on glutamatergic dyshomeostasis. Understanding these mechanisms will highlight an under-studied role for al- tered Mn handling in Alzheimer?s disease, and provide a new target for disease prevention and interventions.